This invention relates to motion detection arrangements and, more particularly, to an improved non-invasive arrangement for detecting the periodic pathwise movement of a ferromagnetic element enclosed within a non-magnetic housing.
Gas meters for domestic and industrial use are typically of the positive displacement diaphragm type. Such a meter conventionally includes a housing with a vertical central partition dividing the housing into two sets of measuring chambers. Within each chamber is a flexible bellows (or convoluted sleeve diaphragm) connected to a central crankshaft by means of connecting rods. The crankshaft actuates a valve system which admits gas in and out of the bellows system. The bellows are caused to expand and contract by the passage of gas through the meter and act in the same manner as pistons to accurately displace a fixed volume of gas for each stroke, or cycle, of the bellows.
One complete cycle of the bellows produces one turn of the crankshaft. In addition to being connected to the valve system, the crankshaft is also connected, through gearing, to a mechanical counter on the front of the meter. This counter is conventionally known as an index and usually contains one circular sweep hand for testing the accuracy of the meter. The dial including this circular sweep hand is commonly called the proving dial. On typical household domestic type meters, the proving dial indicates a flow of two cubic feet of gas through the meter for each turn of its sweep hand. However, due to the connecting gearing, the crankshaft usually makes eighteen turns for each turn of the proving dial sweep hand. Thus, eighteen complete cycles of the bellows are needed for one cycle of the proving dial sweep hand.
Meter accuracy is determined by measuring the actual volume of gas flowing through the meter for each turn of the proving hand. Meters are usually tested at 20% and 100% of their flow rate capacity. Since a domestic type meter normally has a capacity of 250 cubic feet per hour, it requires two minutes and twenty-four seconds for the proving hand to indicate two cubic feet at the 20% capacity rate. This is extremely time consuming when testing meters on a production basis. Accordingly, it is an object of this invention to provide an arrangement for reducing the amount of time required to test meters for accuracy.
It would be desirable to be able to detect a single turn of the crankshaft. At the 20% capacity rate, this would take only eight seconds and at the 100% capacity rate, only one and six-tenths seconds. Therefore, by basing the meter accuracy test on only several turns of the crankshaft (i.e., several cycles of the bellows) considerable time could be saved in proving the meter accuracy. It is therefore another object of this invention to provide an arrangement for accurately detecting the cycles of operation of a meter.
It is a further object to this invention to provide such an arrangement which is non-invasive in nature so that the detecting arrangement does not in any way affect the meter operation.